Juan Diego Valencia, 31, was found guilty this month of second-degree murder and evading police in the September 2012 death of Selene Mayoral.

Mayoral, 24, had recently completed a stint in rehab and was living with her parents in Cudahy, according to Deputy Dist. Atty. Tony Aguilar. But soon after she got a job working at an electronics manufacturer, where she met Valencia, the pair started using meth together, he said.

The letter, which was also signed by Mayoral's father, goes on to describe the anxiety Graciela Mayoral felt as she tried to help her daughter the day she was killed.

"It is difficult to live knowing that she was so afraid, and that all my efforts were unsuccessful," the mother wrote. "She asked me her first question, and she asked him her last. Why?"

On the night she was killed, Mayoral had called and texted her mother, Aguilar had said. She asked her mother to pick her up at a friend's place. She said she didn't want to hang out with Valencia anymore. She told her mother he wouldn't let her leave, Aguilar said.

Hours later, Mayoral called her mother to say she didn't want to be picked up after all. Her mother testified she heard her daughter go quiet and then scream before the call ended.

The callousness of Valencia's crimes led Los Angeles County Superior Court Judge Kelvin Filer to "max out" the sentence, Aguilar said Wednesday.

As a prosecutor who regularly deals with horrific crimes, "I've got to say, this was one that surprised me -- his conduct, before, during and after" the slaying, Aguilar said.

Aguilar said during the trial that a routine traffic stop turned violent when Valencia, high on meth, shot Mayoral, then sped off. He paused at an intersection long enough to push her body from the green Mercury Cougar he was driving, authorities said.

Jurors in the Compton courtroom were shown a graphic video shot by a bystander of Mayoral's limp body tumbling from the car. A trail of lights and sirens follows the car.

Valencia was arrested later that night at his apartment in Hawthorne.

Aguilar contended in his closing arguments that the slaying was a drug-addled display of toughness that followed an afternoon of torture for Mayoral.

Valencia's attorney, Francine Logan, argued that Mayoral's death was an accident -- that when he was pulled over, Valencia had fumbled with a gun in his waistband, and it had gone off.

"What really happened in that car — that's what's at issue," she said during closing statements.